Thermostat Wiring

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jeff43222

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Minneapolis, Minnesota
So I'm playing evening electrician again, and I ran into a puzzler in a thermostat box.

I was called in to replace a 30-year-old thermostat that the HO told me didn't seem to work right. The room, heated with electric baseboard heaters, was either freezing or roasting. She went out to Big Orange (I know, I know ...) and bought a new thermostat (works for either 120V or 240V) for me to install. Here's what I found:

Inside the thermostat box were two 14-2 NM cables. The whites were twisted together, one black was connected to the line terminal, and one was connected to the load terminal. Everything was bonded properly.

The CB protecting the wires was a two-pole 15A breaker. Connected to it were the black and white wires of one 14-2 NM cable. Naturally, this makes me think the baseboard heaters are 240V. But the wiring in the thermostat box sure looked like what I'd expect to find with a 120V circuit.

So I got to wondering if a 240V thermostat is wired with one phase passing through the thermostat and one not (that would explain the wiring in the box). I tend to doubt it, though. Since most people around here heat their homes with gas rather than electricity, I don't have a lot of experience with line-voltage thermostats.

Here's where things get weird. I forgot my meters and only had a voltage detector with me. Both black wires (one line, one load) showed hot. This made no sense, and I figured one of the "hot" wires was actually showing induced voltage. To figure out which was which, I put a light bulb between each hot and the ground wire, and the bulb lit up for each. Now I'm baffled.

What I found in the panel doesn't seem to match up with what I found in the thermostat box. I couldn't find out if the heater was 120V or 240V, and I'm sure not going to guess.

Anyone have any bright ideas?
 
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open the heater up and look at the nameplate...? I've never see 120v baseboard heat. Who ever installed the last T-stat probably only broke 1 leg...
 
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It is common to find 240 volt baseboards hooked up with single pole t-stats

However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.
 
big vic said:
However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.
Click to expand...
I've even been using double pole stats with the contacts connected in series for 120 volt loads, since they are the only line voltage stats that seem to have a marked 'off' position.
 
One of the local box stores does sell 120V baseboard heaters. If it's a low-wattage heater (e.g., 500W), it doesn't make a whole lot of sense to wire it up for 240V and take up two spaces in the panel if the load could easily be carried with a single 14-2 NM cable at 120V.

I just cracked open the heater and found it rated for 240/208. That matches what I found in the panel.

The thermostat the HO bought, according to the instructions, can be used switch-loop style for a 120V load (line2 and load2 leads tied together and not used), and pass-through style for a 240V load.

What confuses me is why the alleged line and alleged load wires in the thermostat box both lit up the light bulb. Only one of them should have.
 
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jeff43222 said:
I just cracked open the heater and found it rated for 240/208. That matches what I found in the panel.

The thermostat the HO bought, according to the instructions, can be used switch-loop style for a 120V load (line2 and load2 leads tied together and not used), and pass-through style for a 240V load.

What confuses me is why the alleged line and alleged load wires in the thermostat box both lit up the light bulb. Only one of them should have.
Click to expand...

Are you talking about a voltage tic? Never trust those... they can be a bit too sensitive at times...
 
No, I know not to trust a voltage tic, as they read induced voltage all the time. Mine was showing both hot, so I used a 60W, 120V light bulb to introduce a high-Z load and find which black wire had the real voltage on it. The bulb lit up each time. ???
 
I think you might be on to something there, stickboy. You're right -- I didn't break the whites apart, so with the CB closed, the "load" wire was actually being fed through the heater back to the thermostat box, right? That would explain why the light lit up both times.

But what still doesn't make sense to me is using a single-pole thermostat for a 240V heater. Seems to me that doing that would have one leg of the heater always hot, but since there are no 120V loads, there would be no reference voltage (neutral) that would allow the heater to operate at 120V.

Sounds to me like a good game plan would be to disconnect the whites, check the voltage between each black/white set to find the line side, and wire up the thermostat properly with both poles going through it.
 
The reason the bulb lit up was the current flowing through the heating element from the other line (the "white")
L2 of breaker to T-stat box, tied through, to heater L2, through element to L1 of heater back to T-stat -
edit to add-Jeff, you typed faster than me!
 
big vic said:
It is common to find 240 volt baseboards hooked up with single pole t-stats

However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.
Click to expand...
I'm not sure I follow. Both ungrounded conductors went dead when I opened the CB, as it was a two-pole breaker. I don't see how the thermostat being single-pole or two-pole would make a difference.
 
jeff43222 said:
I think you might be on to something there, stickboy. You're right -- I didn't break the whites apart, so with the CB closed, the "load" wire was actually being fed through the heater back to the thermostat box, right? That would explain why the light lit up both times.
Click to expand...

That is exactly what happened. You read right through the element.

jeff43222 said:
But what still doesn't make sense to me is using a single-pole thermostat for a 240V heater. Seems to me that doing that would have one leg of the heater always hot, but since there are no 120V loads, there would be no reference voltage (neutral) that would allow the heater to operate at 120V.
Click to expand...
Right again....one leg of a 240v will not do "anything"...it needs the other leg.

jeff43222 said:
Sounds to me like a good game plan would be to disconnect the whites, check the voltage between each black/white set to find the line side, and wire up the thermostat properly with both poles going through it.
Click to expand...
There's nothing wrong with doing something right ;)
 
A 240v load won't operate on 120v, it's the same as a 120v load won't operate without a neutral... Even a 240v Tstat will only break 1 leg in operation, but when you move it to the OFF position, then it will break both legs for servicing....
 
big vic said:
It is common to find 240 volt baseboards hooked up with single pole t-stats

However unless you use a breaker lock, double pole stats should be used with 240 volt heater so both ungrounded conductors are disconnected for servicing.
Click to expand...

Is a t-stat intended to be a disconnect?
 
I know the 240V heater wouldn't operate at 120V, but I guess I just prefer that all ungrounded conductors be dead when you want a device to be off. Probably because of all the switched neutrals I've run into in old houses. Off means off, and to me that means both legs are off.
 
jeff43222 said:
I know the 240V heater wouldn't operate at 120V, but I guess I just prefer that all ungrounded conductors be dead when you want a device to be off. Probably because of all the switched neutrals I've run into in old houses. Off means off, and to me that means both legs are off.
Click to expand...


Exactly, thats why when the Tstat is labeled OFF, its OFF...
 
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